The purpose of this investigation is to determine the effects of aging on epidermal regeneration, a crucial component of repair. Epidermal regeneration will be induced in mice by abrasian. The effects of aging on the overall process of regeneration will be determined by measuring the increases in epidermal wet weight and total protein during epidermal regeneration in mice of varying ages. The contribution of cell number to the increases in epidermal wet weight and total protein will be estimated by measuring the changes in total DNA, and the number of nuclei/mm of epidermis during regeneration in mice of varying ages. Increase in cell number can be brought about by increases in epidermal cell division and/or decrease in epidermal cell loss. The role of epidermal cell division in the increase in cell numbers will be determined by measuring the mitotic index, and the cell labelling index following administration of (3H) thymidine, in mice of varying ages. Increase in cell division can be due to a decrease in the length of the cell cycle and/or to an increase in the growth fraction. The length of the cell cycle will be determined by constructing a % labelled mitoses curve during epidermal regeneration in mice of varying ages. The growth fraction of the epidermis will be estimated by determining the fraction of epidermal cells labelled during continuous administration with (3H) thymidine during epidermal regeneration in mice of varying ages. Finally, the contribution of the decrease in the rate of cell loss to epidermal growth will be determined by measuring the transit time during epidermal regeneration following pulse labelling with (3H) thymidine, both by scintillation counting and autoradiography, in mice of varying age.